The maximum efficiency of a heat engine, given by the Carnot efficiency, can only be achieved if expansion and compression of a working fluid in a variable volume chamber are carried out as nearly isothermally (i.e., at a constant temperature) as possible. The desirability of isothermal expansion and compression is also manifest in a heat pump cycle where it is desired to achieve a coefficient of performance that approaches the Carnot limit. Similarly, a gas compressor can be operated with a minimum amount of work if the compression is carried out isothermally. However, where the volume of working fluid is large, or when the cycle frequency is high, the ideal condition of isothermal expansion and compression is difficult to achieve.
In the past, it has been a practice to use external heat exchangers through which the working fluid is flowed during its expansion and compression. However, external heat exchangers are complex devices which add to the expense and size of the machines. Furthermore, a dead volume is inherent in the use of such external heat exchangers, requiring a larger displacement for a given capacity and pressure ratio. Moreover, the external heat exchangers are sources of axial (thermal shunt) losses due to their cross section.
Isothermalizing of work chambers has always been the goal in the development of highly efficient heat engines such as those employing a Stirling or Ericsson engine. Apparently, some sort of isothermalizing system is employed in the early development of such engines, as indicated in "Napier and Rankine's patent Hot-Air Engines", Mechanics Magazine, No. 1628, Oct. 21, 1854. A patent to Dineen, U.S. Pat. No. 3,220,178, suggests the use of a flexible cloth. In a paper in the Intersociety Energy Conversion Engineering Conference proceedings, Aug. 20, 1973, page 198, entitled "Thermal Losses In Gas-Charged Hydraulic Accumulators" by Professor David R. Otis (University of Wisconsin), the use of a flexible polyurethane foam is suggested. In all these systems, apparently the object was to utilize a flexible material which changed its size and shape in accordance with chamber volume. However, such systems have proved to be very inefficient in actually achieving isothermalization, and the use of heat exchangers is still necessary.